[PATCH] tty: ->signal->tty locking

Fix the locking of signal->tty.

Use ->sighand->siglock to protect ->signal->tty; this lock is already used
by most other members of ->signal/->sighand.  And unless we are 'current'
or the tasklist_lock is held we need ->siglock to access ->signal anyway.

(NOTE: sys_unshare() is broken wrt ->sighand locking rules)

Note that tty_mutex is held over tty destruction, so while holding
tty_mutex any tty pointer remains valid.  Otherwise the lifetime of ttys
are governed by their open file handles.  This leaves some holes for tty
access from signal->tty (or any other non file related tty access).

It solves the tty SLAB scribbles we were seeing.

(NOTE: the change from group_send_sig_info to __group_send_sig_info needs to
       be examined by someone familiar with the security framework, I think
       it is safe given the SEND_SIG_PRIV from other __group_send_sig_info
       invocations)

[schwidefsky@de.ibm.com: 3270 fix]
[akpm@osdl.org: various post-viro fixes]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Jan Kara <jack@ucw.cz>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 141 insertions, 96 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index b3cfc8bc613c..0c856a6f3677 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -126,7 +126,7 @@ EXPORT_SYMBOL(tty_std_termios);
    
 LIST_HEAD(tty_drivers);                 /* linked list of tty drivers */
 
-/* Semaphore to protect creating and releasing a tty. This is shared with
+/* Mutex to protect creating and releasing a tty. This is shared with
    vt.c for deeply disgusting hack reasons */
 DEFINE_MUTEX(tty_mutex);
 EXPORT_SYMBOL(tty_mutex);
@@ -250,7 +250,7 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
                                     "!= #fd's(%d) in %s\n",
                        tty->name, tty->count, count, routine);
                 return count;
-       }        
+        }
 #endif
         return 0;
 }
@@ -259,18 +259,6 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
  * Tty buffer allocation management
  */
 
-
-/**
- *      tty_buffer_free_all             -       free buffers used by a tty
- *      @tty: tty to free from
- *
- *      Remove all the buffers pending on a tty whether queued with data
- *      or in the free ring. Must be called when the tty is no longer in use
- *
- *      Locking: none
- */
-
-
 /**
  *      tty_buffer_free_all             -       free buffers used by a tty
  *      @tty: tty to free from
@@ -614,7 +602,7 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
  *      they are not on hot paths so a little discipline won't do 
  *      any harm.
  *
- *      Locking: takes termios_sem
+ *      Locking: takes termios_mutex
  */
  
 static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
@@ -915,7 +903,7 @@ static void tty_ldisc_enable(struct tty_struct *tty)
  *      context.
  *
  *      Locking: takes tty_ldisc_lock.
- *              called functions take termios_sem
+ *               called functions take termios_mutex
  */
  
 static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
@@ -1267,12 +1255,12 @@ EXPORT_SYMBOL_GPL(tty_ldisc_flush);
  *
  *      Locking:
  *              BKL
- *              redirect lock for undoing redirection
- *              file list lock for manipulating list of ttys
- *              tty_ldisc_lock from called functions
- *              termios_sem resetting termios data
- *              tasklist_lock to walk task list for hangup event
- *
+ *                redirect lock for undoing redirection
+ *                file list lock for manipulating list of ttys
+ *                tty_ldisc_lock from called functions
+ *                termios_mutex resetting termios data
+ *                tasklist_lock to walk task list for hangup event
+ *                  ->siglock to protect ->signal/->sighand
  */
 static void do_tty_hangup(struct work_struct *work)
 {
@@ -1354,14 +1342,18 @@ static void do_tty_hangup(struct work_struct *work)
         read_lock(&tasklist_lock);
         if (tty->session > 0) {
                 do_each_task_pid(tty->session, PIDTYPE_SID, p) {
+                        spin_lock_irq(&p->sighand->siglock);
                         if (p->signal->tty == tty)
                                 p->signal->tty = NULL;
-                        if (!p->signal->leader)
+                        if (!p->signal->leader) {
+                                spin_unlock_irq(&p->sighand->siglock);
                                 continue;
-                        group_send_sig_info(SIGHUP, SEND_SIG_PRIV, p);
-                        group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
+                        }
+                        __group_send_sig_info(SIGHUP, SEND_SIG_PRIV, p);
+                        __group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
                         if (tty->pgrp > 0)
                                 p->signal->tty_old_pgrp = tty->pgrp;
+                        spin_unlock_irq(&p->sighand->siglock);
                 } while_each_task_pid(tty->session, PIDTYPE_SID, p);
         }
         read_unlock(&tasklist_lock);
@@ -1453,6 +1445,14 @@ int tty_hung_up_p(struct file * filp)
 
 EXPORT_SYMBOL(tty_hung_up_p);
 
+static void session_clear_tty(pid_t session)
+{
+        struct task_struct *p;
+        do_each_task_pid(session, PIDTYPE_SID, p) {
+                proc_clear_tty(p);
+        } while_each_task_pid(session, PIDTYPE_SID, p);
+}
+
 /**
  *      disassociate_ctty       -       disconnect controlling tty
  *      @on_exit: true if exiting so need to "hang up" the session
@@ -1469,31 +1469,35 @@ EXPORT_SYMBOL(tty_hung_up_p);
  *      The argument on_exit is set to 1 if called when a process is
  *      exiting; it is 0 if called by the ioctl TIOCNOTTY.
  *
- *      Locking: tty_mutex is taken to protect current->signal->tty
+ *      Locking:
  *              BKL is taken for hysterical raisins
- *              Tasklist lock is taken (under tty_mutex) to walk process
- *              lists for the session.
+ *                tty_mutex is taken to protect tty
+ *                ->siglock is taken to protect ->signal/->sighand
+ *                tasklist_lock is taken to walk process list for sessions
+ *                  ->siglock is taken to protect ->signal/->sighand
  */
 
 void disassociate_ctty(int on_exit)
 {
         struct tty_struct *tty;
-        struct task_struct *p;
         int tty_pgrp = -1;
+        int session;
 
         lock_kernel();
 
         mutex_lock(&tty_mutex);
-        tty = current->signal->tty;
+        tty = get_current_tty();
         if (tty) {
                 tty_pgrp = tty->pgrp;
                 mutex_unlock(&tty_mutex);
+                /* XXX: here we race, there is nothing protecting tty */
                 if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
                         tty_vhangup(tty);
         } else {
-                if (current->signal->tty_old_pgrp) {
-                        kill_pg(current->signal->tty_old_pgrp, SIGHUP, on_exit);
-                        kill_pg(current->signal->tty_old_pgrp, SIGCONT, on_exit);
+                pid_t old_pgrp = current->signal->tty_old_pgrp;
+                if (old_pgrp) {
+                        kill_pg(old_pgrp, SIGHUP, on_exit);
+                        kill_pg(old_pgrp, SIGCONT, on_exit);
                 }
                 mutex_unlock(&tty_mutex);
                 unlock_kernel();        
@@ -1505,19 +1509,29 @@ void disassociate_ctty(int on_exit)
                         kill_pg(tty_pgrp, SIGCONT, on_exit);
         }
 
-        /* Must lock changes to tty_old_pgrp */
-        mutex_lock(&tty_mutex);
+        spin_lock_irq(&current->sighand->siglock);
         current->signal->tty_old_pgrp = 0;
-        tty->session = 0;
-        tty->pgrp = -1;
+        session = current->signal->session;
+        spin_unlock_irq(&current->sighand->siglock);
+
+        mutex_lock(&tty_mutex);
+        /* It is possible that do_tty_hangup has free'd this tty */
+        tty = get_current_tty();
+        if (tty) {
+                tty->session = 0;
+                tty->pgrp = 0;
+        } else {
+#ifdef TTY_DEBUG_HANGUP
+                printk(KERN_DEBUG "error attempted to write to tty [0x%p]"
+                       " = NULL", tty);
+#endif
+        }
+        mutex_unlock(&tty_mutex);
 
         /* Now clear signal->tty under the lock */
         read_lock(&tasklist_lock);
-        do_each_task_pid(current->signal->session, PIDTYPE_SID, p) {
-                p->signal->tty = NULL;
-        } while_each_task_pid(current->signal->session, PIDTYPE_SID, p);
+        session_clear_tty(session);
         read_unlock(&tasklist_lock);
-        mutex_unlock(&tty_mutex);
         unlock_kernel();
 }
 
@@ -2337,16 +2351,10 @@ static void release_dev(struct file * filp)
          * tty.
          */
         if (tty_closing || o_tty_closing) {
-                struct task_struct *p;
-
                 read_lock(&tasklist_lock);
-                do_each_task_pid(tty->session, PIDTYPE_SID, p) {
-                        p->signal->tty = NULL;
-                } while_each_task_pid(tty->session, PIDTYPE_SID, p);
+                session_clear_tty(tty->session);
                 if (o_tty)
-                        do_each_task_pid(o_tty->session, PIDTYPE_SID, p) {
-                                p->signal->tty = NULL;
-                        } while_each_task_pid(o_tty->session, PIDTYPE_SID, p);
+                        session_clear_tty(o_tty->session);
                 read_unlock(&tasklist_lock);
         }
 
@@ -2443,9 +2451,9 @@ static void release_dev(struct file * filp)
  *      The termios state of a pty is reset on first open so that
  *      settings don't persist across reuse.
  *
- *      Locking: tty_mutex protects current->signal->tty, get_tty_driver and
- *              init_dev work. tty->count should protect the rest.
- *              task_lock is held to update task details for sessions
+ *      Locking: tty_mutex protects tty, get_tty_driver and init_dev work.
+ *               tty->count should protect the rest.
+ *               ->siglock protects ->signal/->sighand
  */
 
 static int tty_open(struct inode * inode, struct file * filp)
@@ -2467,12 +2475,13 @@ retry_open:
         mutex_lock(&tty_mutex);
 
         if (device == MKDEV(TTYAUX_MAJOR,0)) {
-                if (!current->signal->tty) {
+                tty = get_current_tty();
+                if (!tty) {
                         mutex_unlock(&tty_mutex);
                         return -ENXIO;
                 }
-                driver = current->signal->tty->driver;
-                index = current->signal->tty->index;
+                driver = tty->driver;
+                index = tty->index;
                 filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
                 /* noctty = 1; */
                 goto got_driver;
@@ -2547,17 +2556,16 @@ got_driver:
                         filp->f_op = &tty_fops;
                 goto retry_open;
         }
+
+        mutex_lock(&tty_mutex);
+        spin_lock_irq(&current->sighand->siglock);
         if (!noctty &&
             current->signal->leader &&
             !current->signal->tty &&
-            tty->session == 0) {
-                task_lock(current);
-                current->signal->tty = tty;
-                task_unlock(current);
-                current->signal->tty_old_pgrp = 0;
-                tty->session = current->signal->session;
-                tty->pgrp = process_group(current);
-        }
+            tty->session == 0)
+                __proc_set_tty(current, tty);
+        spin_unlock_irq(&current->sighand->siglock);
+        mutex_unlock(&tty_mutex);
         return 0;
 }
 
@@ -2747,7 +2755,7 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
  *
  *      Copies the kernel idea of the window size into the user buffer.
  *
- *      Locking: tty->termios_sem is taken to ensure the winsize data
+ *      Locking: tty->termios_mutex is taken to ensure the winsize data
  *              is consistent.
  */
 
@@ -2774,8 +2782,8 @@ static int tiocgwinsz(struct tty_struct *tty, struct winsize __user * arg)
  *      Locking:
  *              Called function use the console_sem is used to ensure we do
  *      not try and resize the console twice at once.
- *              The tty->termios_sem is used to ensure we don't double
- *      resize and get confused. Lock order - tty->termios.sem before
+ *              The tty->termios_mutex is used to ensure we don't double
+ *      resize and get confused. Lock order - tty->termios_mutex before
  *      console sem
  */
 
@@ -2880,25 +2888,28 @@ static int fionbio(struct file *file, int __user *p)
  *      leader to set this tty as the controlling tty for the session.
  *
  *      Locking:
- *              Takes tasklist lock internally to walk sessions
- *              Takes task_lock() when updating signal->tty
  *              Takes tty_mutex() to protect tty instance
- *
+ *              Takes tasklist_lock internally to walk sessions
+ *              Takes ->siglock() when updating signal->tty
  */
 
 static int tiocsctty(struct tty_struct *tty, int arg)
 {
-        struct task_struct *p;
-
+        int ret = 0;
         if (current->signal->leader &&
             (current->signal->session == tty->session))
-                return 0;
+                return ret;
+
+        mutex_lock(&tty_mutex);
         /*
          * The process must be a session leader and
          * not have a controlling tty already.
          */
-        if (!current->signal->leader || current->signal->tty)
-                return -EPERM;
+        if (!current->signal->leader || current->signal->tty) {
+                ret = -EPERM;
+                goto unlock;
+        }
+
         if (tty->session > 0) {
                 /*
                  * This tty is already the controlling
@@ -2908,24 +2919,18 @@ static int tiocsctty(struct tty_struct *tty, int arg)
                         /*
                          * Steal it away
                          */
-
                         read_lock(&tasklist_lock);
-                        do_each_task_pid(tty->session, PIDTYPE_SID, p) {
-                                p->signal->tty = NULL;
-                        } while_each_task_pid(tty->session, PIDTYPE_SID, p);
+                        session_clear_tty(tty->session);
                         read_unlock(&tasklist_lock);
-                } else
-                        return -EPERM;
+                } else {
+                        ret = -EPERM;
+                        goto unlock;
+                }
         }
-        mutex_lock(&tty_mutex);
-        task_lock(current);
-        current->signal->tty = tty;
-        task_unlock(current);
+        proc_set_tty(current, tty);
+unlock:
         mutex_unlock(&tty_mutex);
-        current->signal->tty_old_pgrp = 0;
-        tty->session = current->signal->session;
-        tty->pgrp = process_group(current);
-        return 0;
+        return ret;
 }
 
 /**
@@ -2937,7 +2942,7 @@ static int tiocsctty(struct tty_struct *tty, int arg)
  *      Obtain the process group of the tty. If there is no process group
  *      return an error.
  *
- *      Locking: none. Reference to ->signal->tty is safe.
+ *      Locking: none. Reference to current->signal->tty is safe.
  */
 
 static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
@@ -2995,7 +3000,7 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
  *      Obtain the session id of the tty. If there is no session
  *      return an error.
  *
- *      Locking: none. Reference to ->signal->tty is safe.
+ *      Locking: none. Reference to current->signal->tty is safe.
  */
 
 static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
@@ -3214,14 +3219,11 @@ int tty_ioctl(struct inode * inode, struct file * file,
                         clear_bit(TTY_EXCLUSIVE, &tty->flags);
                         return 0;
                 case TIOCNOTTY:
-                        /* FIXME: taks lock or tty_mutex ? */
                         if (current->signal->tty != tty)
                                 return -ENOTTY;
                         if (current->signal->leader)
                                 disassociate_ctty(0);
-                        task_lock(current);
-                        current->signal->tty = NULL;
-                        task_unlock(current);
+                        proc_clear_tty(current);
                         return 0;
                 case TIOCSCTTY:
                         return tiocsctty(tty, arg);
@@ -3321,7 +3323,7 @@ static void __do_SAK(struct work_struct *work)
         
         if (!tty)
                 return;
-        session  = tty->session;
+        session = tty->session;
         
         /* We don't want an ldisc switch during this */
         disc = tty_ldisc_ref(tty);
@@ -3834,9 +3836,52 @@ int tty_unregister_driver(struct tty_driver *driver)
         cdev_del(&driver->cdev);
         return 0;
 }
-
 EXPORT_SYMBOL(tty_unregister_driver);
 
+dev_t tty_devnum(struct tty_struct *tty)
+{
+        return MKDEV(tty->driver->major, tty->driver->minor_start) + tty->index;
+}
+EXPORT_SYMBOL(tty_devnum);
+
+void proc_clear_tty(struct task_struct *p)
+{
+        spin_lock_irq(&p->sighand->siglock);
+        p->signal->tty = NULL;
+        spin_unlock_irq(&p->sighand->siglock);
+}
+EXPORT_SYMBOL(proc_clear_tty);
+
+void __proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
+{
+        if (tty) {
+                tty->session = tsk->signal->session;
+                tty->pgrp = process_group(tsk);
+        }
+        tsk->signal->tty = tty;
+        tsk->signal->tty_old_pgrp = 0;
+}
+
+void proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
+{
+        spin_lock_irq(&tsk->sighand->siglock);
+        __proc_set_tty(tsk, tty);
+        spin_unlock_irq(&tsk->sighand->siglock);
+}
+
+struct tty_struct *get_current_tty(void)
+{
+        struct tty_struct *tty;
+        WARN_ON_ONCE(!mutex_is_locked(&tty_mutex));
+        tty = current->signal->tty;
+        /*
+         * session->tty can be changed/cleared from under us, make sure we
+         * issue the load. The obtained pointer, when not NULL, is valid as
+         * long as we hold tty_mutex.
+         */
+        barrier();
+        return tty;
+}
 
 /*
  * Initialize the console device. This is called *early*, so